Hydrogels are water-swellable or water-swollen materials having a structure defined by a crosslinked network of hydrophilic homopolymers or copolymers. The hydrophilic homopolymers or copolymers may or may not be water-soluble in free form, but in a hydrogel are rendered insoluble (but swellable) in water due to covalent, ionic, or physical crosslinking. In the case of physical crosslinking, the linking may take the form of entanglements, crystallites, or hydrogen-bonded structures. The crosslinks in a hydrogel provide structure and physical integrity to the network.
Hydrogels have been made from a variety of hydrophilic polymers and copolymers. Poly(ethylene glycol), poly(vinyl pyrrolidone), polyacrylamide, poly(hydroxyethyl methacrylate), and copolymers of the foregoing, are examples of polymers that may be used to make hydrogels. Hydrogels have also been made from biopolymers such as chitosan, agarose, hyaluronic acid and gelatin, in addition from semi-interpenetrating network (“IPN”) hydrogels and gelatin crosslinked with poly(ethylene glycol) diacrylate.
Poly(vinyl alcohol) (“PVA”) has been studied extensively for potential biomedical applications. PVA hydrogels can be produced, for example, from an aqueous solution via repeated freezing and thawing cycles that increase the order of the crystals, changing the dissolution properties, mesh size, and diffusion properties of the polymer.
Hydrogels have shown promise in biomedical and pharmaceutical applications, due, in part, to their high water content and rubbery or pliable nature, which may mimic natural tissue and may facilitate the release of bioactive substances at a desired physiological site. For example, hydrogels have been used or proposed for use in a variety of tissue treatment applications, including implants, tissue adhesives, bone grafts as well as in meniscus and articular cartilage replacement. Hydrogels may also act as a carrier for delivering bioactive substances including drugs, peptides, and proteins to a physiological site.
However, many biomedical applications require that the implanted article possess different characteristics, such as mechanical and chemical properties, at different locations or surfaces of the article. Thus, there is a need to provide hydrogel materials and articles that present different characteristics at different locations of the implanted article.